Semiconductor based light sources, such as light emitting diodes (LEDs), have long life and high luminous efficiency, but vary in several respects, including color, intensity, and aging. Different colors of LEDs will not have equal intensities and will age differently. Even two LEDs of the same color, processed in the same manufacturing batch, can vary slightly in all of these respects. When combining multiple source colors of light to create a target color, it is necessary to be able to accurately to determine the chromaticity of each source color and the intensity of the source colors relative to each other. The less accurate the chromaticity and intensity measurements of the source colors, the less accurately the target color will be produced. This inaccuracy can cause two LED light sources that are both set to the same target color to have different appearances.
The LEDs of each light source can be measured with a spectrometer to determine the exact chromaticity and intensity. Target colors will then be reproduced accurately at the temperature that the LEDs were measured at. As the temperature increases LED output intensity is reduced and the wavelength may shift. The same changes take place permanently over thousands of hours of usage. Since one of the selling features of many LED light sources is the long life, it would seem important to maintain accuracy in produce target colors over that lifetime.
The inherent variations between LEDs, along with the need for precise color and intensity of light output over time, necessitate a mechanism to account for and control these differences. Current methods measure temperature or current density over time and attempt to mathematically predict the adjustments necessary to produce consistent light output. For example, a small subset of LEDs may be measured, and algorithms are used to estimate the LED characteristics for the rest of the LED array. These methods, however, are only approximations and fall far short of the goal of consistent, high-quality color output. Each LED in the array may be measured separately, but this is difficult for a large array of LEDs. Even so, points between known temperature data points or known current density points are still extrapolated.